Inequality condition for real numbers

CAT 2023 Slot 2 · QA · Medium · Algebra

Any non-zero real numbers x,yx, y such that y3y \neq 3 and xy<x+3y3\frac{x}{y} < \frac{x+3}{y-3}, will satisfy the condition

  1. A.

    if y>10y > 10, then x>y-x > y

  2. B.

    if x<0x < 0, then x>y-x > y

  3. C.

    if y<0y < 0, then x>y-x > y

  4. D.

    xy<yx\frac{x}{y} < \frac{y}{x}

Answer

B

Explanation

We are given: x+3y3xy>0\frac{x+3}{y-3} - \frac{x}{y} > 0 y(x+3)x(y3)y(y3)>0\frac{y(x+3) - x(y-3)}{y(y-3)} > 0 xy+3yxy+3xy(y3)>0    3(x+y)y(y3)>0\frac{xy + 3y - xy + 3x}{y(y-3)} > 0 \implies \frac{3(x+y)}{y(y-3)} > 0

So x+yy(y3)>0\frac{x+y}{y(y-3)} > 0.

If x<0x < 0:

  • If y>3y > 3, then y(y3)>0    x+y>0    y>x>0y(y-3) > 0 \implies x + y > 0 \implies y > -x > 0. In this case, x<y-x < y.
  • If 0<y<30 < y < 3, then y(y3)<0    x+y<0    x>yy(y-3) < 0 \implies x + y < 0 \implies -x > y.
  • If y<0y < 0, then y(y3)>0    x+y>0    x>yy(y-3) > 0 \implies x + y > 0 \implies x > -y. But since x<0x < 0 and y<0y < 0, y>0-y > 0, so x>yx > -y is impossible.

Thus, if x<0x < 0, yy must lie in (0,3)(0, 3), which forces x>y-x > y.

Hence, Option B ("if x<0x < 0, then x>y-x > y") is correct.

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